1. Field of the Invention
The present invention relates to a large-capacity inverter device comprising a plurality of parallel-connected semiconductor switching elements and a smoothing capacitor arranged beside them.
2. Description of the Prior Art
A conventional inverter device of this type has a bridge circuit in each constituent phase, the upper arm and the lower arm of the bridge circuit comprising a plurality of semiconductor switching elements connected in parallel. FIG. 1 shows the construction of one of the phases. Respective input ends of semiconductor switching elements 1 for an upper arm are connected by a positive-side short-circuiting bar 5 to the positive pole of a smoothing capacitor 3. Similarly, input ends of semiconductor switching elements 2 for a lower arm are connected by a negative-side short-circuiting bar 6 to the negative pole of the smoothing capacitor 3.
Respective output ends of the elements 1, 2 are connected to an output-side short-circuiting bar 7, and then make outputs to the outside from an output terminal at the other end of the short-circuiting bar 7.
FIG. 2 shows the directions of current flow by arrows when the device illustrated in FIG. 1 is turned on. The arrows on the one-dot chain line indicate overall current flow, while the arrows on the broken lines indicate shunted current flow across the respective elements.
FIGS. 3 and 4 are front and plan views, respectively, showing examples of the arrangement of the components of the above-described inverter device. As illustrated in these drawings, semiconductor switching elements 1 for an upper arm and semiconductor switching elements 2 for a lower arm are each disposed in parallel connection on a cooling member 4, and a smoothing capacitor 3 is arranged laterally virtually on the longitudinal extension of the parallelly disposed elements 1, 2.
The so arranged elements 1, 2 and the smoothing capacitor 3 are connected together by short-circuiting bars as shown in FIGS. 5A, 5B, 5C and 6. In these drawings, the short-circuiting bars are wired in a tree-like fashion so that the distance between each element 1 or 2 and the smoothing capacitor 3 in a direct current circuit may be equal.
More particularly, the arranged elements 1 and the smoothing capacitor 3 are connected together by two (first) short-circuiting bars 5, a second short-circuiting bar 5a, and a third short-circuiting bar 5b. The short-circuiting bars 5, 5a, and 5b are wired in a tree-like fashion so that the distance between each element 1 and the smoothing capacitor 3 in a direct current circuit may be equal.
Also, the arranged elements 2 and the smoothing capacitor 3 are connected together by two (first) short-circuiting bars 6, a second short-circuiting bar 6a, and a third short-circuiting bar 6b. The short-circuiting bars 6, 6a and 6b are wired in a tree-like fashion so that the distance between each element 2 and the smoothing capacitor 3 in a direct current circuit may be equal. As indicated by arrows in FIG. 6, currents flow across the tree-like wiring bar structure comprised of the short-circuiting bars 5, 5a, and 5b and 6, 6a, and 6b in opposite directions to each other, and wiring inductance between each element 1 or 2 and the smoothing capacitor 3 becomes non-uniform. Therefore, when elements 1, 2 are turned on, an imbalance of current arises because of the difference in wiring inductance. In other words, when current flows in the direction of arrows as shown in FIG. 6, wiring inductances transiently constitute resistances, thus forming some elements with a high flowability of current and some other elements with a low flowability of current. Furthermore, the tree-like wiring bar structure poses the problem that it is complicated, requires a large number of parts which are difficult to assemble, involves high cost, and attends to be unstable in quantity.
The occurrence of imbalance in current necessitates the selection of the capacities of the elements 1, 2 based on the maximum value of current. The expected excessive increase in capacity induces higher costs, and the larger element capacities require larger dimensions. Furthermore, the range of applying the rated capacity of the inverter is restricted, since the maximum capacity is reduced.
In regard to the above problem associated with wiring inductance, the influence of offsetting of wiring inductance may be decreased by increasing the distances a, b, and c between the respective current paths shown in FIGS. 5A, 5B and 5C. In this case as well, the dimensions would be increased to an unnecessary extent, thus leading to an oversize of the device. Furthermore, the tree-like wiring bar structure poses the problem that it is complicated, requires a large number of parts which are difficult to assemble, involves high costs, and tends to be unstable in quality.